Mechanisms linking obesity and hypertension to ischemic heart disease (IHD) are not clearly understood. Most patients with IHD suffer from atherosclerotic lesions that limit coronary blood flow; however, more than one-third of all patients have no observable obstruction on angiography. The flow-limiting reductions in coronary arterial diameter in such patients are thought to be due to functional abnormalities (i.e. enhanced vasoconstriction; impaired vasodilation). Apelin is formed in adipose tissue and secreted into the circulation where it has multiple effects on blood vessel function, depending on the vascular bed. Activation of the apelin/APJ receptor-signaling pathway has vasodilatory effects in important vascular beds (e.g. pulmonary) and is generally thought to play a protective role in the cardiovascular system. Our long-term goal is to understand the mechanisms by which apelin regulates vasomotor tone in health and disease. Studies from our lab indicate that apelin causes endothelium- and NO-dependent relaxation of healthy coronary arteries. By contrast, in coronary arteries from hypertensive animals we found that apelin failed to cause arterial relaxation and, instead, inhibited endothelium-dependent relaxation to acetylcholine. Our central hypothesis is that the apelin/APJ receptor axis does not provide a protective vasodilator role in the coronary circulation in hypertension, but rather presents a net vasoconstrictor activity in this vascular bed. It is further hypothesized, based on strong preliminary data, that this adverse effect of apelin is due to inhibition of NO production in hypertensive coronary endothelial cells. It is proposed that the apelin-mediated inhibition of NO synthesis in hypertensive coronary arteries is sufficient to functionally impair the arteries and create a local environment that favors vasoconstriction. This is significant, as it would predictably increase the risk of coronary vascular dysfunction in conditions with elevated plasma apelin levels, such as obesity. Two specific aims are designed to investigate our hypotheses: In Aim #1, we will address the hypothesis that the distinction between the effect of apelin in normotensive vs hypertensive coronary arteries is secondary to changes in APJ receptor levels or location within the vessel wall. Quantitative measurements of gene and protein expression, as well as imaging studies to determine the spatial localization of APJ receptors in the coronary arterial wall will be used to accomplish this aim. In Aim #2, a combination of molecular biological, functional, and imaging techniques will be used to identify which signaling pathway(s) coupling APJ receptor activation to NO production is disrupted in hypertensive coronary arteries. The knowledge gained from these studies will have strong positive impact by increasing our understanding of the mechanisms that control coronary vasomotor tone under hypertensive conditions. Such information may provide new targets for therapeutic interventions in patients with IHD, as well as inform potential therapeutic strategies based on improving hemodynamics with apelin and apelin-like APJ receptor agonists already in clinical development.